The use of wireless devices to transfer data is becoming increasingly prevalent. Two wireless devices, a source and destination, may communicate directly with each other. For example, a cellular phone communicates with a cellular base station to provide voice connectivity to the public switched telephone network. Another example is a wireless local area network where wireless client devices communicate with an access point.
The data transfer can be one-way, from the source to the destination, or two-way, where data traffic also travels from the destination to the source. The maximum rate of throughput of data between the source and the destination, hereafter referred to simply as throughput, is an important quantity that characterizes the performance of the data channel between the source and destination. It is desirable to increase the throughput as much as possible.
In certain cases, it is desirable for the source and destination wireless devices to route or relay their data transmission through intermediate wireless devices. These intermediate devices are generally referred to as routers, repeaters, or relays. The intermediate devices may be needed when the source and destination are not within transmission range of each other. Other wireless devices can be used to relay the data forming a chain from the source to the destination, with each link in the chain being a wireless device is in transmission range of the originating device. The number of wireless devices in the chain may be as small as one, or as large as need be. FIG. 1 shows a source marked S 100 communicating to a destination marked D 170, via the intermediate wireless routers marked I1 110, I2 120, I3 130, I4 140, I5 150, and I6 160. The two-way arrows between the routers indicate a link that has been formed between those routers. For example, node I1 110 can communicate directly with node I2 120, and vice-versa. However, to reach I4 140, node I1 must communicate through nodes I2 120, and I3 130.
A half-duplex transceiver is a wireless device that can either transmit or receive, but not both at the same instant of time. An example of a half-duplex transceiver is that provided by a personal computer with an IEEE 802.11 interface. Wireless devices may also consist of a separate transmitter and receiver at each node. If transmitter and receiver can transmit and receive data from another node at the same time, the wireless device is called full-duplex transceiver. An example of a full-duplex transceiver is an IS-95 CDMA cell phone. FIG. 2 illustrates the difference between half-duplex and full-duplex transceivers. As can be seen, full duplex transceivers can transmit and receive at the same time, while half-duplex transceivers either receive or transmit. However, half-duplex transceivers are generally cheaper, and more easily available. Therefore, many devices use half-duplex transceivers. Often, two half-duplex transceivers are included in each device, such that one half-duplex transceiver can transmit, while the other one receives data.
One prior art method of implementing of a chain or a mesh of wireless devices is used in wireless ad-hoc networks, as described by MANET, DARPA SURAN, etc. Two wireless devices communicate with each other by leveraging peer wireless devices to route or relay the data. The applications envisioned in such networks included battlefield (military) communications and mobile (civilian) networks.
Another prior art method that forms a chain of wireless devices is the transceivers and full-duplex repeaters that constitute the infrastructure of Metricom's network. The repeaters sit atop street lamp poles and relay information from client user modems to wired access points and vice-versa.
Wireless devices must be equipped with antennas in order to receive and transmit data. Omni-directional antennas transmit or receive signals with equal strength in all directions in the horizontal plane. If the antennas are not omni-directional, they are known as directional antennas, and these have radiation patterns that are not circularly symmetric in the horizontal plane.
The directional properties of one hypothetical directional antenna that is connected to a source S 300 in the horizontal plane are illustrated in FIG. 3. As can be seen, the source S 300 can only communicate with destination D2 320, and not with any of D1 310, D3 330, D4 340, D5 350, and D6 360 since the antenna does not transmit or receive in those directions. One prior art method, used by Radiant, Plc., employs directional links on both transmitters and receivers among wireless devices forming a chain.
Directional links are overly restrictive for many forms of terrestrial communication since they permit communication only a certain fixed direction at any given instant of time. For applications such as the prior art mentioned above, MANET and DARPA SURAN, directional links (in contrast to omni-directional links), are not usable since they would prevent formation of an ad-hoc mesh network between wireless devices in arbitrary directions. In these applications, there may not be prior knowledge of the direction between a given wireless device and another wireless device. Therefore aiming antennas with directional links presents a difficulty, especially when setting up communication using a chain of wireless devices that may be reconfigured based on changes in network of wireless devices including the introduction or malfunction of one of the devices.